Case study: measurement of Q and cumulative attenuation from
VSP data

Gary F. Margrave

ABSTRACT

The measurement of attenuation in seismic data is described and
analyzed. The measurement problem is defined as the estimation of the
attenuation parameter Q or the estimation of the related quantity CA
(cumulative attenuation) or both. Two very different estimation
techniques are described: the spectral-ratio method (SRM), which is
wellknown, and the dominant-frequency method, which is mostly new
here. The strengths and weaknesses of both methods are discussed and
the extension to CA is given. It is demonstrated that CA estimates are
more stable than Q estimates when attenuation is weak. The
application of these techniques is demonstrated on a zero-offset VSP
with a vibroseis source. Using the shallowest receiver (2185ft) as a
reference, attenuation estimates were obtained for all receivers at
depths equal to or greater than 5000ft. Consistent estimates were
obtained from both the SRM and the DFM but it is demonstrated that any
residual upcoming waves in the downgoing wave cause considerable
error. The possibility of extending these measurements to the earth’s
surface by assuming the reference wave there is the Klauder wavelet is
examined. The results are plausible and seem appropriate to apply to
surface recordings for bandwidth enhancement.